In digital speech communication systems, the transmission link causes transmission errors which deteriorate the quality of the speech signal in the receiver. In order to provide protection against interference due to the transmission errors, an error correction coding is utilized for the signal transmitted on the transmission link in order to correct the errors that occur, or at least to detect them. In many digital communication systems, speech coding methods that take the special characteristics of speech into account and slow down transmission rate are required due to capacity limits on the transmission link. The Pan-European mobile communication system GSM (Global System for Mobile Communications), for example, uses a RPE-LTP (Regular Pulse Excitation--Long Term Prediction) speech coder, which utilizes both short and long term prediction. Log Area Ratio (LAR), RPE, and LTP parameters for speech transmission are produced by the RPE-LTPE coding. In communication systems, such as the GSM which use speech coding, the most important speech coding parameters as far as the quality of speech is concerned, and, in addition, the most significant bits from most of speech coding parameters of less importance, are protected by error correction coding. In addition, the most important bits are associated with an error verification code used to detect errors which the actual error correction coding is not able to correct. The receiver, however, is not able to detect all transmission errors that have not been corrected. For this reason, defective speech frames, i.e., speech frames whose speech coding parameters include erroneous parameter values, are occasionally input to the speech decoder of the receiver. As a consequence, the quality of the received speech signal is deteriorated by these erroneous parameters.
If the transmission link is of a fairly high quality, the error correction capability is usually adequate to correct the errors which are caused by the transmission link in the speech signal. If the transmission link, instead, is of poor quality, due to a low field strength in a border area between two base stations of a mobile communication network, or in an area shadowed by terrain, for example, the correction capability of the error correction and the error detection capability may be exceeded. Consequently, it is possible that some of defective frames are interpreted as correct. If such defective frames are used in the speech decoding, the defective frames cause disturbance to the audible speech, and thus, deteriorate the quality of the speech signal. Disturbances caused by defective speech parameters which are classified as correct in the receiver, but which, in fact, are defective, constitute a problem in communication systems, especially during a relatively poor quality transmission link.